With increasing recognition of environmental protection, environmental regulations are gradually becoming stricter. It is believed that decreasing the sulfur content in gasoline or diesel fuel is one of the most important measures to improve the air quality because the sulfur contained in the fuel adversely affects the performance of the catalytic converter of automobiles and vehicles. The sulfur oxides present in the exhaust gas from automobile engines inhibit the activity of the noble metal catalyst in the converter and poisons the catalyst irreversibly. Gases released from an ineffective or poisoned converter comprise uncombusted non-methane hydrocarbons, nitrogen oxide and carbon monoxide, all of which easily form photochemical smogs when catalyzed by sunlight.
In China, most sulfur contained in gasolines comes from heat processed gasoline, which is mainly catalytic cracking gasoline. Therefore, decreasing the sulfur content in the cracking gasoline would facilitate reducing sulfur content of these gasolines. The current standard for gasoline product is GB 17930-2006 “Motor Vehicle Gasoline,” which further restricts the sulfur content of gasoline and requires that by Dec. 31, 2009 the sulfur content of gasoline be lowered to 50 ppm. This circumstance means that catalytic cracking gasoline must be desulfurized to a great degree in order to meet the environmental requirement.
When lowering the sulfur content of motor vehicle fuel, changes in olefin content which leads to a reduction of octane number (including Research Octane Number, ROM and Motor Octane Number, MON) should be avoided so to retain the combustion characteristics of the motor vehicle fuel. Generally, the negative change on the olefin content is caused by the hydrogenation reaction induced upon removal of thiophene compounds (including thiophene, benzothiophene, alkylthiophene, alkylbenzothiophene and alkyldibenzothiophene). Further, the loss of aromatic hydrocarbons in the cracking gasoline due to saturation under hydrogenation condition should also be avoided. Therefore, the most desirable approach is to desulfurize the gasoline while retaining its octane number.
On the other hand, both hydrodesulfurization and hydrogenation of unsaturated hydrocarbons consume hydrogen, which increases the operational cost of the desulfurization. Accordingly, there is a need for a method of desulfurization without consuming large volumes of hydrogen, thereby providing a more economical method for treatment of cracking gasoline or diesel fuel.
Traditionally, a fixed-bed process has been used for desulfurization in a liquid phase. However, this process is disadvantageous in the homogeneity of reaction and the regeneration of the material. Compared with the fixed-bed process, a fluidized-bed process is advantageous given wider applications prospects in the future because of better heat transfer and pressure drop. In this context, a fluidized-bed reactor is usually provided with granular reactants. However, for most reactions, the granular reactants do not have sufficient abrasion-resistance. Accordingly, it is of great significance to find a granular reactant, or adsorbent, with both excellent abrasion-resistance and desulfurization performance.
Chinese Patents CN 1110931A and CN 1151333A describe a new absorbing composition comprising zinc oxide, silicon dioxide, colloidal oxide and an accelerant and a process for making the same. In the process, fluidizable particles are produced by a pressure forming technique, and the particle pore volumes are increased by adding to the colloid a pore-forming agent which becomes flammable when heated. The particles prepared by this process are comparatively big, and the particle size is within the range of about 100 to about 300 micron, which is not most favorable for the fluidization process.
U.S. Pat. No. 6,150,300, Chinese Patents CN 1355727A and CN 1258396 disclose a granular adsorbent composition comprising a mixture of zinc oxide, silica, alumina, nickel or cobalt in a reduced state. The adsorbent is made by first mixing silica, alumina and zinc oxide under a shearing force, preparing the solid particle through a granulating machine, and impregnating nickel after drying and calcinating the particle. These patents do not disclose the physical-chemical properties, particularly the abrasion-resistance of the adsorbent, although the adsorbent disclosed in these patents reportedly show good desulfurization performance.
Chinese Patent CN 1422177A describes a process for making an adsorbent for removal of the sulfide contained in cracking gasoline. The process comprises the steps of impregnating an adsorbent carrier comprising zinc oxide, expanded perlite and alumina with accelerant metal such as cobalt and nickel, and subsequently reducing the accelerant at an appropriate temperature. The abrasion-resistance of the adsorbent can be improved by adjusting the level of zinc oxide and binder (particularly alumina from Disperal and Vista Dispal, Condea) in the adsorbent. Chinese Patent CN 1627988A further discusses in detail the major compounds produced under the reaction conditions. Chinese Patents CN 1856359 and CN 1871063 disclose that the physical-chemical properties of particles prepared by spray drying method in this patent are more suitable for fluidized-bed, as well as the particulars of adsorbents with similar constituents and a process of making the same.
In preparing these adsorbents, the strength of adsorbents is improved by adding clay. However, because the clay has no pores, the pore volume of the adsorbents tends to be smaller and thus the activity of the adsorbent decreases. Therefore, it is most desirable to have an adsorbent with high pore volume and high strength.
In view of the above, it is desirable to provide a new adsorbent composition for removing sulfur from cracking gasoline or diesel fuel and a process of making the adsorbent.